We have obtained evidence that the growth-promoting action of prolactin (PRL) on the mucosal epithelium of the pigeon crop-sac involves synergism with an insulin-like growth factor (IGF). Furthermore, PRL stimulates the liver to secrete this synergistic activity, which we have called synlactin. Slices of liver from pregnant or lactating rats (but not from adult males or virgin females) secrete synlactin activity in vitro as measured by the crop-sac assay. Thus, the factor is secreted by the rat liver when the organ is influenced by high levels of placental lactogen (in pregancy) or pituitary PRL (lactation). The liver inculation medium samples that had synlactin activity in the crop-sac (from pregnant or lactating rats) also had mammary mitogenic activity in an assay involving explants of pregnant rat mammary glands. We propose to determine which cells in the mammary gland respond to the hepatic mammary mitogen. Mammary explants from virgin or pregnant rats will be incubated in liver incubation medium containing no synlactin activity (from male or virgin female liver) or with the activity (from pregnant or lactating liver). The mitogenic effect will be assessed by autoradiographic analysis of the incorporation of 3H-thymidine into cell nuclei. The effects of the liver incubation media will be compared with human IGF-I, PRL and insulin, and the effects of these hormones in different combinations will be determined. The proliferative effects of the hepatic factor and the other hormones (alone and in combination) will also be tested on rat mammary ductal and alveolar cells in a collagen gel matrix. The specificity of the hepatic mammary mitogenic effect will be evaluated by incubating explants of other tissue (salivary gland, kidney, thymus) in liver incubation medium with or without the activity. We will also investigate whether the liver participates in the regulation of mammary growth in vivo. This will be accomplished by infusing PRL and/or GH into the hepatic portal vein of virgin female rats using osmotic minipumps attached to PE-10 cannulae. This experiment should show whether PRL or GH can stimulate the liver to secrete factors that promote mammary ductal or lobulo-alveolar growth. Attempts will be made to purify and characterize the hepatic factor with synlactin activity from liver incubation medium, and to develop an RIA for it so that other aspects of its physiology can studied.